1. Timers and Event Counters
Embedded Systems

2. In These Notes . . .
We learn the basics of the Timer/Counter peripheral
Called timers by Renesas
We examine how to set up the timers for different operation:
Timer mode
Event counting mode
Pulse Width Modulation (PWM) mode
One-shot timer mode
We then examine how to use a microcontroller in these modes
Embedded Systems

3. Timer/Counter Introduction
Current Count
Reload Value
Presettable Binary Counter
Events
Reload or
÷2 or RS
PWM
Clock
Interrupt
Common peripheral for microcontrollers
Based on pre-settable binary counter, enhanced with configurability
Count value can be read and written by MCU
Count direction can often be set to up or down
Counter’s clock source can be selected
Counter mode: count pulses which indicate events (e.g. odometer pulses)
Timer mode: clock source is periodic, so counter value is proportional to elapsed time (e.g. stopwatch)
Counter’s overflow/underflow action can be selected
Generate interrupt
Reload counter with special value and continue counting
Toggle hardware output signal
Stop!
Embedded Systems

4. Timer A Block Diagram
Each timer has one input, which is selectable from several different sources.
Embedded Systems

5. High-Level Timer A Block Diagram
Timer A devices will be the most frequently used
Flexible – can be cascaded to create larger timers (i.e. 32 bits long)
Embedded Systems

6. Timer A Mode Register
To use the timer, you must set up how you wish to use it (i.e. via TA0MR). After that, the mode register has different settings depending on bits 1 and 0.
Embedded Systems

7. Timer A “Data” Register
Embedded Systems

8. Count Start Register
Once the timer has been loaded with a value, start it counting.
Embedded Systems

9. Counter Mode Count pulses representing events Odometer example
Measure total distance your car has traveled
Events are wheel rotations, measured with magnetic sensors (dirt-proof!)
Distance traveled = counter value * ”
Assume 16” tire radius. Tire circumference = 2pr = ”
Will limited range of 16 bit counter be a problem?
100.53” * = miles
Yes. So need to extend range in software.
Enable overflow interrupt for the timer
Create an ISR to count overflows
Embedded Systems

10. TAiMR in Event Counting Mode
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11. Up/Down Flag The default is that the timer will count down.
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12. Trigger Select Register
You can set the trigger pulse of Timers A1 to A4
Embedded Systems

13. Example – Setting-up Event Mode
#define TIME_CONFIG 0x01 /* val to load into timer mode reg
||||||||_ TMOD0,TMOD1: EVENT COUNTR MODE
||||||____ MR0: NO PULSE OUTPUT
|||||_____ MR1: COUNT FALLING EDGES
||||_______MR2: USE UP/DOWN FLAG
|||_______ MR3: = 0 IN EVENT COUNTER MODE
||________ TCK0: RELOAD TYPE
|__________TCK1: BIT NOT USED */
#define CNTR_IPL 0x // TA2 priority interrupt level
#define LED p7_ // LED port on MSV1632 board
#define LED_PORT_DIRECTION pd7_2 // LED port dirn on MSV1632 board
void init() {
ta2 = 100; //e.g for an automated packaging line, 100 items per case
// the following procedure for writing an Interrupt Priority Level follows
// that as described in the M16C data sheets under 'Interrupts'
_asm (" fclr i") ; // turn off interrupts before modifying IPL
ta2ic |= CNTR_IPL; // use read-modify-write instruction to write IPL
ta2mr = TIME_CONFIG;
_asm (" fset i");
ta2s = 1; //start counting }
Embedded Systems

14. TAiMR in Timer Mode
Embedded Systems

15. Example – Timer & Event Mode
The maximum amount of time one can count using two timers is:
Cascade Timer A0 to Timer A1
Use the 1/32 clock (bits 7 and 6 of TA0MR set to ’10’)
Set both timer values to xFFFF
Timer A1 output generates an interrupt
This would create a timer event which would interrupt the CPU every ______ seconds (almost __ hours!)
Embedded Systems

16. Pulse-Width Modulation
Often need to generate an analog value: motor speed, servo motor position, incandescent light dimmer, adj. switch-mode power supply
Analog circuits have some disadvantage
Generating the voltage accurately – circuits drift with temperature and time
Analog power amplifiers are inefficient (P = E*I)
Analog signals are much more sensitive to noise than digital signals
PWM provides a digital encoding of an analog value
Now circuits are digital: more efficient, less sensitive to noise, temperature, aging effects
PWM signal characteristics
Modulation frequency – how many pulses occur per second (fixed)
Period – 1/(modulation frequency)
On-time – amount of time that each pulse is on (asserted)
Duty-cycle – on-time*period
Adjust on-time (hence duty cycle) to create encoded analog value
Embedded Systems

17. Pulse-Width Modulation
We rely on low-pass filtering to “decode” (convert) this high-frequency PWM signal to an analog DC value
Variable speed motor control: Motor has inertia (will coast if power is turned off)
Incandescent Lamp Dimmer: Human eye has persistence of vision (retina averages signals above 30 Hz), and filament takes time to cool down and stop emitting visible light
Adjustable switch-mode power supply: Rely on inductor and capacitor to maintain current and voltage when not drawing current from battery
Many Timer/Counter peripherals can also generate PWM signal
For example
Count up
Set PWM output when counter value reaches 0
Reset PWM output when counter value reaches n
When counter reaches , overflow and continue counting
Read Michael Barr’s Embedded Systems Programming article “Pulse Width Modulation” for more PWM information
Embedded Systems

18. Timer A PWM Description
Embedded Systems

19. Timer A Mode Register for PWM
Embedded Systems

20. One-Shot Flag
Embedded Systems